Resin transfer molding (RTM) has been used to manufacture netshape parts for a wide range of industries ranging from aerospace to automobile. In its most basic form, a fiber mat is cut, shaped, and/or stamped into a preform which is placed in a mold cavity. After the mold is closed and clamped, a resin is injected into the cavity. As the resin flows through the preform, it expels the air and wets the preform. When the cavity has been filled, the temperature of the mold is increased and the resin starts curing. The part is removed from the mold after the resin has cured.
It will be appreciated that, while the present invention and much of the following discussion relates specifically to RTM, the invention has equal utility with other forms of liquid composite molding (LCM). As used herein, LCM is intended to refer to any molding process where a formable material flows through a porous media, and where the flow of the formable material can be represented by mathematical equations of the type disclosed herein.
The structural requirements often dictate that the molded parts should not contain incompletely filled areas, or so-called dry spots. A dry spot can appear either because of air entrapment, use of low feed pressure, or both, or the like.
Dry spots are usually formed in an LCM part when some of the air in the mold is trapped in an area of the cavity from where it cannot escape through an open vent. Areas of the mold where air is trapped unconnected to an open vent during the mold filling process (or simulation thereof) may also be referred to herein as an incipient dry spot. The trapped air may or may not ultimately escape through an open vent during the filling process, depending upon the gas pressure in the incipient dry spot and the resin pressure in the immediate neighborhood of the dry spot.
It will thus be understood that, for the purposes of this discussion, unless referenced in connection with a finished part, all references to "incipient dry spot" or "dry spot" are used interchangeably to mean an incompletely filled area in the flow domain at a specific point in time in the mold filling, or simulated mold filling, process.
One way to enhance resin flow to reduce or eliminate dry spots is to apply a vacuum to the mold prior to injecting the resin in the mold (also known as "passive vacuum"). However, there is always some residual air. Moreover, as the resin flashes into the cavity, it may release vapors. The trapped air and vapor are both referred to as "trapped air" in the subsequent discussions.
The trapped air hinders the resin flow into the unfilled region. Pressure in the unfilled region increases as more resin flows into that region, thereby decreasing the difference between the feed pressure and the pressure in the unfilled region. Since this pressure acts as the driving force for the resin flow, the resin stops flowing into the unfilled region when the pressure in the unfilled region becomes equal to the feed pressure. Under these circumstances, the cured part will contain a dry spot.
Alternatively or additionally, a vacuum can be applied while the resin is injected in the mold (also known as "active vacuum"). However, even during this process, some of the residual air may get entrapped in the preform. Application of an active vacuum also often results in considerable wastage of resin.
Often a combination of passive and active vacuums are utilized to minimize the occurrence of dry spots while limiting resin wastage. First, a vacuum is pulled in the mold, the vent ports are closed, and the resin is injected under passive vacuum. The residual air in the mold is pushed into the unfilled region and pressure in the unfilled region increases. The difference between the feed pressure and the pressure in the unfilled region, which acts as the driving force to the resin flow, decreases, resulting in a decreasing resin flow rate. When the flow rate becomes negligible, the vents are opened to apply an active vacuum. Some of the trapped air then escapes through the vents. More resin is then injected under passive vacuum. This sequence of alternating between closing the vent ports and opening the vent ports is known as a "pack and bleed" sequence. A pack and bleed sequence is typically repeated until no more air comes out of the open vents. It will be appreciated by those skilled in the art that a simulation of the LCM process must therefore include the capability of handling changes in pressure within the cavity which occur as a result of the introduction of pack and bleed sequences.
Another factor which may affect the formation of dry spots, and, therefore, must be accounted for in any simulation, is racetracking. Racetracking is the phenomenon in which the resin flows faster through the higher permeability regions of the preform, such as edges and bends, than through the middle of the undeformed preform. A consequence of racetracking is that the higher permeability regions are filled before the lower permeability regions, which may result in air entrapment and, possibly, dry spots in the finished part.
Other factors which alone, or in combination, affect the formation of dry spots include the part geometry, the location of the feed and vent ports, and the feed conditions.
Mold flow simulations have been recognized as a potentially valuable tool in predicting resin flow rates and patterns. However, a more thorough understanding of the process of air entrapment and dry spot formation, and in particular air entrapment during mold filling in RTM, is desirable in order to optimize mold design, reduce developmental time and costs, and increase the efficiency of the molding process.